1. Field of the Invention
This invention relates to an in-mold foam molding of a vinylidene chloride type resin, and expandable particles and foam particles convenient for preparation of the molding, more particularly to a novel vinylidene chloride type resin in-mold foam molding having a broad cross-section and a free shape which can be utilized as such for insulating material boards, shock absorbing molded vessels, etc., and a series of foaming techniques sufficient for completion thereof.
2. Description of the Prior Art
In recent years, abundant studies have been made about techniques for expansion of synthetic resins. As a result, a large number of synthetic resins have been made expandable, and individual technical fields have developed depending on the type of resins used. However, a complete technique for obtaining good foam molding which is satisfactory with respect to cross-section, shape and dimension has not been developed for vinylidene chloride type resins. Accordingly, there exists no foam molding having a broad cross-sectional shape and a plate area which can be used as such for an insulating material board.
The following reasons for this behavior may generally be contemplated for vinylidene chloride type resins:
(1) The processing temperature for melt processing of the resin is so close to the decomposition temperature at which decomposition will proceed that thermal decomposition of the resin will occur in the extrusion processing step;
(2) Decomposition of the resin is markedly accelerated when the resin comes into contact with a metal such as iron or copper under a temperature around the melting point of the resin;
(3) Due to high gas barrier property of the resin, the resin can be impregnated only with a small amount of a blowing agent, and therefore it can expand little by heating;
(4) The dependency of the rheological properties of the resin on the temperature around the expansion temperature is so great that the expanding conditions are difficult to control.
Thus, under the present situation, it is very difficult to obtain a highly expanded homogeneous foam molding without causing thermal decomposition, and no satisfactory foaming technique has been developed.
In the prior art, concerning foam moldings of vinylidene chloride type resins and preparation thereof, for example, proposals have been made about extrusion expansion by choice of special chemical blowing agents (Japanese Patent Publications Nos. 3968/1964 and 16419/1967, U.S. Pat. No. 2,948,048). However, these foam moldings are low in expansion ratio, which is about 2- to 3-fold, and the final product cross-sections are limited to small cross-sectional shapes, as represented only by artificial bamboo blind, artificial bamboo blind core, ornamental threads, etc. The purpose of expansion is also no more than controlling the surface lustre or imparting flexibility.
Alternatively, as a technique for high expansion by use of a physical blowing agent, the method has been also known, in which finely divided vinylidene chloride type resin is mixed with a physical blowing agent, and the resultant mixture is extrusion expanded at lower temperatures (about 120.degree. to 150.degree. C.) to give an extruded foam with a density of about 240 Kg./m.sup.3 or less and cell sizes of about 0.1 to 1 mm (U.S. Pat. No. 3,983,080). However, according to this method, it is difficult to control thermal decomposition and thermal decomposition of the resin proceeds, whereby it is possible to continue extrusion expansion to give only strand-shaped foamed extrudate with uneven surface and markedly irregular cell sizes.
Further, expandable unicellular spherical particles with diameters of about 1 to 50 um including a volatile liquid blowing agent within a thermoplastic resin copolymer have also been proposed (Japanese Patent Publication No. 26524/1967 and Japanese Laid-open Patent Publication No. 59168/1974). In this proposal, the definition of the thermoplastic resin is inclusive of copolymers of vinylidene chloride with acrylonitrile or butyl acrylate, and there is the description in a part of the examples that a foam-like product can be formed through fusion of the particles by effecting heating expansion. However, the above foam and the foam of the present invention differ in principle of expansion, the structures of particles and foam, performance and therefore use. The technical difference is explained to draw clearly a line of demarcation between both.
First, as the greatest difference in technique, the foam of the present invention is prepared by expanding particles containing a blowing agent impregnated (dissolved) therein to give multi-cellular foam particles enriched in recovery and resilience. The multi-cellular particles are in turn formed into a mass according to the in-mold molding technique. As a consequence, it is possible to obtain a foam with a high closed cell percentage excellent in mechanical strength. In contrast, the expandable particles according to the aforesaid proposal are so called micro-baloons, in which liquid blowing agent is included within small baloon-like entities made of a resin, and therefore, even when these may be fused by heating expansion, the resultant foam molding is a mass consisting of units of baloon-like unicellular bubbles, with a low close cell percentage, and also inferior in mechanical characteristics. Further, the microbaloons are used primarily as a mixture with inks or paints for applying relief patterns on wall papers, etc., and their particle sizes are very small, as small 1 to 50 um, as mentioned above. Therefore, when it is attempted to mold the particles, they cannot be filled uniformly within a mold or steam cannot be passed to inner portions of the molding. Thus, they are basically different from the expandable resin particles of the present invention in that they cannot be formed into a mass according to the in-mold expansion molding as intended by the present invention. Also, the expandable resin particles of the present invention can be extrusion expanded to give a homogeneous good extrusion expanded board with a large cross-section and high closed cell percentage. On the other hand, when the expandable resin particles according to the above proposal are attempted to be extrusion expanded, the shells of the resin containing the liquid blowing agent will be broken to form an unhomogeneous mixture of the resin and the blowing agent, which can be discharged through the nozzle only to result in evaporation of the blowing agent, with the resin substantially failing to be expanded. Also, in this respect, the expandable resin particles of this invention are fundamentally different from the expandable particles according to the above proposal.